Weird "pre-flash" behavior on Fenix UC-35

buckeyes7

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Feb 2, 2015
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Hey guys, new poster, but long time reader here. I have had the PD35 for around a year now, and have been very happy with it. I just ordered a UC35 from Amazon last week, and it arrived the other day.

I've noticed that when pressing the tail cap button in halfway (but firmly) in momentary mode, there is a quick flash of light, and then the light comes on as normal. This only happens in the two (or sometimes, but rarely, three) lowest modes, and the flash is always the same intensity as the mode the light it in. I've googled and read about preflash problems, but they seem to be talking about having the light in low mode, and it flashing in the highest mode on startup. My light flashes, but the intensity of the flash is the same as the mode.

I've tried to recreate this on my PD35 with no luck.

Additionally, I've noticed that it doesn't happen every time. It may happen about half of the time. It seems less likely to happen if the light has been on in the last few seconds. Fro example, If I press the momentary button 4 times with about 1 second in between presses, it will preflash on the first press, but not the others. Then if I wait about 5 seconds and repeat, it will do it again only on the first press.

I am wondering if this is a sign of a bad component (maybe a capacitor that has to 'charge' up on startup, but has some residual charge when the light was on recently?) in the regulating circuit that I need to worry about failing soon.

Can anybody with this light try (in eco mode is fine) and let me know if they see similar behavior?

For a $100 light, I am thinking it shouldn't do that and am thinking about trying to get it replaced before it fails all together?

Thanks!
 

buckeyes7

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Feb 2, 2015
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Just wanted to post an update.

I returned the "defective" light (the one exhibiting what I called "preflash.") Today I received the replacement light, and was pretty upset to find that it does the exact same thing. So I guess this is normal.

Ugh.

Oh well. It's a really nice light otherwise, but this flash is super annoying.

Nobody else has this issue?
 

CelticCross74

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Man that sucks! I adore my UC35 and have had zero issues with it. I know its annoying to have to do but Fenix has an awesome record when it comes to making things right, just switch it again and request that whoever is sending you the replacement test the switch before it is sent. Just tried recreating the problem you are having with my UC35 but mine functions perfectly...could be maybe it has to do with a possible bad shipment of UC35's that got sent to X online seller? I have seen that before. Sounds like a bad switch to me which is almost unheard of for Fenix but hey with mass produced lights anything is possible. Buy my lights from GoingGear UC35 included and have had perfect luck with everything I have gotten from them its almost as if manufacturers make sure their more popular retailers get the batches that have had any issues worked out of...

Yes for a $100 light I would send it back to...I myself would personally contact Fenix about the issue...good luck.
 

CelticCross74

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after re reading this thread I think its just a sensitive switch issue. I can get my UC35 to do the same thing if I press the switch with differing pressures. Im so used to my UC35 though Ive got how much pressure to use and how much travel to traverse to work the momentary function and how much to get the switch to latch on etc...good luck
 

Str8stroke

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Yup, believe it or not, it is fairly common! I have had several lights that do this. I have sold or returned all but one. I tried everything, but switch pressure is a interesting idea. I was pretty sure it was cause by the circuit.

My 47 Quark Pro does it, but because of the UI I can start in high or low and I actually love everything else about the light. But, if you like starting in low mode, just aim the light at your hip and then after its on move it so you don't blind yourself.
 

metasyntax

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A little bit of a late response, but I was searching around and found this thread last night after noticing the exact same behavior on my PD35. When in lower modes, it briefly flashes that mode before coming on for real. It almost seems like the lower the mode, the longer the flash. Only seems to happen on the first activation over some period of time (turn it on again within 30 seconds and it doesn't happen... wait longer and it will). I haven't tried adjusting the switch to get rid of it. Doesn't bother me much, because the longer flash on my MT1A (when it's in low mode) has trained me to overlook these things. :)
 

hiuintahs

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The "preflash" is output overshoot. The electronic driver has a feedback to keep it regulated. However when first turning on, the output ramps up very fast. For the most part you can't tell that its ramping up. If the stability is set right, then it will not overshoot. But if its not quite right, then it will overshoot before it drops into regulated output. It won't be a reliability issue in my opinion. It's just kind of annoying. I've had lights overshoot. Some designers build in a slower ramp at start of a tenth of a second or so.

Anyhow, I think that is what is going on.
 

WarRaven

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Oct 24, 2013
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Not to take anything away from the logical post above mine but,

Has the folks experiencing this tried checking how tight switch was in tail cap by chance?

I'm wondering if it's slightly loose and shifting each time it's pressed after sitting and relaxed.
Jmtc.
 

metasyntax

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Has the folks experiencing this tried checking how tight switch was in tail cap by chance?

I just tried tightening mine. It was a touch loose, but tightening it up didn't change anything... still get a bit of a low flash when turning on. Oh well, it was worth a shot. Like I said, doesn't bother me much.
 

WarRaven

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I just tried tightening mine. It was a touch loose, but tightening it up didn't change anything... still get a bit of a low flash when turning on. Oh well, it was worth a shot. Like I said, doesn't bother me much.
Was worth a shot, sorry not the right one though.
 

CelticCross74

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this thread is back! Okay after much maintenance on my UC and PD's I have found that taking the light apart head, tube and tail then cleaning ALL connection points with rubbing alcohol has worked wonders. They were all actually kinda dirty from the factory which is of course disappointing but hey. Always check tailring tightness hell I even go as far as taking the tail switches apart and cleaning then putting them back together. So far all the tail switches are "crisper" seem to activate quicker. Might help Idk just thought Id throw that in esp since Ive got PD35#5(the new TAC) coming next week. Im sure It will need all points wiped down as well. I even go as far as taking rubbing alcohol to the battery terminals helps sometimes
 
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